1. Field of the Invention
The present invention relates to an amplification optical fiber and a fiber optic amplifier including the same.
2. Related Background Art
A fiber optic amplifier is an optical device for amplifying a plurality of channels of signals so as to compensate for their loss when they propagate through an optical transmission line in an optical communication system; and comprises an amplification optical fiber and a pumping light source. Namely, if such a fiber optic amplifier is disposed at a predetermined position on an optical transmission line, then it amplifies the power of signals introduced to the amplification optical fiber to which pumping light of a predetermined wavelength is supplied from the pumping light source. Known as such a fiber optic amplifier are an amplifier in which an optical fiber whose optical waveguide region is doped with a rare-earth element is utilized as the amplification optical fiber (hereinafter referred to as rare-earth element doped fiber optic amplifier) and an amplifier utilizing Raman amplification (hereinafter referred to as Raman amplifier).
The Raman amplifier is described, for example, in literature 1, i.e., S. Hamidi, et al., Electronics Letters, Vol. 28, No. 18, pp. 1768-1770 (1992); literature 2, i.e., E. Desurvire, et al, Electronics Letters, Vol. 19, No. 19, pp. 751-753 (1983); and literature 3, i.e., Y. Aoki, et al., Electronics Letters, Vol. 19, No. 16, pp. 620-622 (1983). On the other hand, the optimization of refractive index profile of amplification optical fiber is described, for example, in literature 4, i.e., M. J. Holmes, et al, Electronics Letters, Vol. 26, No. 22, pp. 1873-1874 (1990).
The inventor has studied the above-mentioned conventional techniques and, as a result, have found problems as follows. Namely, amplification optical fibers employed in conventional fiber optic amplifiers ensure a single mode at each of the signal wavelength (e.g., 1.55 xcexcm) and the wavelength of pumping light (e.g., 1.48 xcexcm), thereby propagating only the fundamental mode (LP01 mode) for each of them. In such a case, however, the effective area of an amplification optical fiber at the wavelength of pumping light (dependent on the power distribution in the fundamental mode of pumping light) becomes smaller than the effective area of the amplification optical fiber at the signal wavelength (dependent on the power distribution in the fundamental mode of signals).
Therefore, the conventional fiber optic amplifiers have failed to yield a sufficient amplification gain due to the difference between the respective effective areas of amplification optical fiber at two wavelengths different from each other. In particular, pumping is insufficient at a marginal portion of the core distanced from the center in the region contributing to amplification in the amplification optical fiber, whereby there is a possibility that the signal fails to be amplified or is absorbed on the contrary. Near the core center, in contrast, the amplification efficiency is higher than that at the marginal portion of core, thus yielding a higher signal power, whereby nonlinear optical phenomena are likely to occur. From such a viewpoint, there has been a limit to the enhancement of amplification gain. Further, a semiconductor laser light source (pumping light source) for outputting only the fundamental mode of pumping light is hard to yield a higher output, whereby there has been a limit to the enhancement of amplification gain from this viewpoint as well.
Though a plurality of semiconductor laser light sources may be used so as to enhance the power of pumping light supplied to the amplification optical fiber, this configuration is unfavorable since it complicates the fiber optic amplifier and raises the manufacturing cost.
For solving the problems such as those mentioned above, it is an object of the present invention to provide an amplification optical fiber comprising a structure which can yield a higher amplification gain, and a fiber optic amplifier including the same.
The fiber optic amplifier according to the present invention includes any of the Raman amplifier and rare-earth element doped fiber optic amplifier; and comprises, at least, a pumping light source for supplying pumping light of a predetermined wavelength and an amplification optical fiber.
Preferably, when the fiber optic amplifier is a Raman amplifier in particular, the amplification optical fiber (the amplification optical fiber according to the present invention) has one end for inputting pumping light of a predetermined wavelength and has, at an effective length, a cutoff wavelength which is shorter than a wavelength of signals but longer than the wavelength of pumping light. If the fiber optic amplifier is a rare-earth element doped fiber optic amplifier, in contrast, then it is preferred that the amplification optical fiber (the optical fiber according to the present invention) have one end for inputting pumping light and have, at a length of 2 m, a cutoff wavelength longer than the wavelength of pumping light.
Such an amplification optical fiber propagates only the fundamental mode for signals, but can propagate at least the second-order mode together with the fundamental mode for the pumping light. Namely, since the power distribution of a higher-order mode of pumping light is added to the power distribution of fundamental mode of pumping light, a power distribution approximating the power distribution of fundamental mode of signals is obtained. Consequently, the difference between the respective effective areas at the signal wavelength and at the pumping light wavelength effectively decreases, thereby enhancing the amplification gain. Since the amplification optical fiber can be pumped not only near the center of its core but also in the marginal portion of core contributing to amplification, the amplification optical fiber is suitable for enhancing the amplification gain in view of this point as well.
In an optical transmission system including the amplification optical fiber according to the present invention, the fundamental mode of signals propagating through the amplification optical fiber has a group velocity which differs from that of the fundamental mode and second-order mode of pumping light to such an extent that signals do not deteriorate an SN ratio thereof to a level which is problematic in terms of operation of the optical transmission system. This is because of the fact that, even when there is a difference in group velocity between the fundamental mode of signals and the fundamental mode of pumping light, the amplification gain is enhanced upon pumping by a higher-order mode of pumping light if the group velocity difference between the fundamental mode of signals and the higher-order mode of pumping light is small.
As mentioned above, the amplification optical fiber according to the present invention can propagate the second-order mode in addition to the fundamental mode for pumping light. Under a condition securing a single-mode operation in which only the fundamental mode is propagated for signals, the third- and higher-order modes of pumping light usually attenuate greatly. Hence, if the third- and higher-order modes of pumping light are kept from propagating, then the amplification gain can fully be enhanced.
Preferably, the amplification optical fiber according to the present invention includes a polarization-maintaining optical fiber for preserving respective polarization states of signals and pumping light upon propagation thereof. In this case, the respective polarization states of signals and pumping light coincide with each other, thereby enhancing the amplification gain.
In the case of Raman amplifier, it is preferred that the amplification optical fiber employed therein comprise a plurality of glass layers radially laminated in succession, among which at least a glass layer having the highest refractive index is doped with a material which enhances a Raman gain coefficient. Such a Raman amplifier can further improve the utilization efficiency of pumping light.
When the amplification optical fiber according to the present invention is applied to a rare-earth element doped fiber optic amplifier, the amplification optical fiber preferably has a cutoff wavelength of 1480 nm or more. Preferably, this cutoff wavelength is shorter than a wavelength of signals propagating through the amplification optical fiber.